1. Field of the Invention
This invention relates to porous refractory hard metal compositions. More particularly, this invention relates to a porous refractory hard metal composition formed from a substantially interwoven matrix composition comprising a refractory hard metal and a metal compound.
2. Description of the Prior Art
Refractory hard metals, such as TiB.sub.2 or ZrB.sub.2 are useful in applications where a material is needed possessing chemical inertness and/or mechanical strength, particularly at elevated temperatures. Porous materials comprising refractory hard metals would be particularly useful for filtering molten metals or, especially at elevated temperatures, corrosive materials such as molten salts, because of the chemical inertness and mechanical strength of a refractory hard metal.
The interest in the use of TiB.sub.2 as an electrode or electrode surface, for an electrolytic cell has generated great interest in ways to economically produce such a product in a form that could be used as a cathode. For example, U.S. Pat. No. 4,353,885 discloses forming TiB.sub.2 by vapor phase reaction. Additionally, U.S. Pat. No. 3,016,288 discloses an aluminothermic process of making boron compounds and alloys.
Walton et al in "Cermets From Thermite Reactions", published January, 1959, in Volume 42 of the Journal of the American Ceramic Society, at pages 40-49, discusses thermite reactions for forming materials such as ZrSi.sub.2, by the reduction of either ZrSiO.sub.4 or ZrO.sub.2 and SiO.sub.2, and TiB.sub.2, by the reduction of TiO.sub.2 and B.sub.2 O.sub.3.
One factor which has limited the utilization of refractory hard metals is the high cost normally involved in producing such materials. Thus, in order to obtain the benefits of such a material, it can be seen that there is a great need for a process which would provide refractory hard metals such as TiB.sub.2 at an economically feasible cost.
An article entitled "Displacement Reactions in the Solid State", by R. A. Rapp et al, published May 1973, in Volume 4 of Metallurgical Transactions at pages 1283-1292, points out the different morphologies which can result from the addition of a metal or metal alloy to an oxide mixture. The authors show that some additions result in layers of metal or metal oxides while others form aggregate arrangements which may be lamellar or completely interwoven. An example of an interwoven structure is shown for the reaction of Fe with NiO which produces a NiFe alloy/FeO structure. The authors suggest that interwoven-type microstructures should be ideal for the transfer of stresses and resistance of crack propagation and demonstrated that such were not fractured by rapid cooling. The authors further suggested that such an interwoven structure would be useful in the preparation of porous electrodes for fuel cells or catalysts for reactions between gases by selective dissolution of either the metal or the oxide phase.
In my parent U.S. patent application Ser. No. 604,913, there is disclosed a novel composition in which a refractory hard metal, such as TiB.sub.2, is formed in an interwoven matrix with a metal compound by a displacement reaction using first and second reactants which form the refractory hard metal and a third reactant which forms a metal compound. These reaction products are substantially uniformly dispersed in the resulting interwoven matrix composition. This invention relates to a porous refractory hard metal composition formed from such a substantially interwoven matrix and a method of making such porous refractory hard metal compositions by removing the metal compound from the interwoven matrix composition.